In the petroleum industry, lengths of pipe are threaded together to form a drill-string (i.e., a length of series-connected pipes). However, with the increasing popularity of extended-reach drilling, multi-lateral wells, and horizontal well applications, the stress and bending moments that are placed on the treaded connections in a drill-string, are increased. The increased number of pipe connections and increased stresses contribute to an increased chance of a downhole failure of the drill-string. The cost of repairing a single downhole failure may exceed one million dollars.
Cold root-rolling is the process of burnishing the root radius of a freshly-cut or previously-cut thread in a rotary shouldered connection. A hardened roll, similar in profile to the thread being burnished, is forced into contact with the root radius of the thread. Pressure is applied to cause the hardened roll to penetrate into the cut surface of the root radius. This deforms and cold-works the material, imparts an improved surface finish to the thread, and compacts and displaces the grains of the root material.
Industry experience has suggested that cold root-rolling may increase the fatigue life of a threaded connection from three to five times over a similar untreated connection under the same working conditions. Some studies have noted laboratory results suggesting that an improvement in fatigue life of up to twenty-seven times may result from the cold root-rolling process. See, e.g., Knight, M. J., Brennan, F. P. and Dover, W. D., “Fatigue Life Improvement of Threaded Connections by Cold Rolling”, Journal of Strain Analysis, vol. 40, pp. 83-93 (Sep. 30, 2004). These various studies have attributed the increase in fatigue life to one or more of the following factors:
(1) Cold root-rolling creates a thin zone of residual compressive stress in the root region of the thread. This residual compressive stress offsets the tensile stresses produced in service, and lowers the overall stress in the critical stress region of the thread root.
(2) The burnishing effect of the smooth and hardened roll causes small scratches and ridges left by the thread-cutting operation to flatten into a more-uniform surface. These scratches may have small tip radii, and are believed to be the source of considerable stress concentrations. As a result, these scratches appear to be the crack propagation points for fatigue failures.
(3) Scratches provide prime locations for chemical erosion. The microscopic surface of a scratch is very jagged and porous. This exposes a large surface area, and numerous molecular bonding sites to the corrosive effects of liquids and gasses in a drill-string environment. Burnishing smoothes this surface, and reduces out-croppings and inclusions. It tends to reduce the area of the surface, and densely compresses the same.
(4) Cold root-rolling has a work-hardening effect of the surface of the material. On an atomic scale, compressive displacement of the crystalline lattice within the steel grain structure is believed to cause the crystal structure to change from a repetitive and uniform atomic structure to one with many dislocations in the pattern. These dislocations are believed to cause the crystal structure to interlock, and to become more resistant to further deformation. This increased resistance to further deformation helps to prevent cracks from starting, and helps to arrest microscopic cracks from growing into structural flaws that might threaten the integrity of the joint. In laboratory studies, cracks that have occurred in cold-rolled joints have exhibited a significantly-lower crack aspect ratio (i.e., the ratio of crack length to crack depth). A 30%-50% reduction in this ratio means that cracks that have occurred in cold-rolled joints are more likely to be deep and short, rather than long and shallow. A long and shallow crack is more likely to lead to a sudden and complete structural failure of the joint. A deep crack that partially penetrates the section wall is detectable via the pressure drop of circulating drilling fluids, and allows for an early recovery of a damaged drill-string prior to a complete structural failure of the joint.
Because of the foregoing advantages, cold root-rolling is now commonly performed on many freshly-cut and re-cut threads used on drill-string pipes used in the petroleum industry. It is a money-saving process. It can dramatically increase the fatigue life of each rotary-shouldered connection in a typical drill-string. It can also reduce the frequency of repairing connections in the field, and of having to fish for downhole failures.
One line of cold root-rolling products is available from Cutting Tools, Inc., 5050 Ashley Court, Houston, Tex. 77041.
Accordingly, it would be generally desirable to provide an improved device for cold root-rolling freshly-cut or re-cut threads on an object.